1. Field of the Invention
The present invention is related to an optical transmitting terminal, and particularly to an optical transmitting terminal used in an optical fiber transmission system in which light amplifiers are used as optical repeaters.
2. Description of the Related Art
In the optical communication system in which NRZ optical pulses are transmitted on an optical transmission path in which many optical repeaters are interconnected, it is known that the transmission characteristic is deteriorated by the accumulation of minute polarization dependent factors inherent in the optical amplifiers. As a technique for suppressing such polarization dependent factors to improve the transmission characteristic, a high-speed electro-optic polarization scrambler is effective, in which polarization is scrambled at a speed equal to or higher than a frequency which is the same as the bit rate (reference: F. Heismann, et al., IEEE Photon, Technol., Lett., vol. 6, no. 9, pp 1156-1158, 1994). Although it was reported that the best transmission characteristic is provided when the frequency of the high-speed electro-optic polarization scrambler is the same as the bit rate and synchronous with the modulation signal (reference: F. Heismann, IOOC'95, Paper FD1-2, 1995), the study by the present inventors proved that the characteristic might be significantly degraded depending on the phase of the signal applied to the high-speed electro-optic polarization scrambler.
FIG. 10 is an example of the result of a measurement carried out with 5.3 Gbps, in which the abscissa represents the relative signal phase of the high-speed polarization scrambler driving signal, and the ordinate represents the relative Q-value after a transmission over a long distance (8800 km). It is seen that the relative Q-value after the transmission changes as much as about 3 dB if the phase of the scrambler driving signal is changed in the range of about 180 ps (pico seconds) corresponding the one bit period. This change, if replaced by code error rate, is a large change of about six orders from about 10.sup.-7 to 10.sup.-13, and thus it is understood that a strict phase adjustment of the high-speed electro-optic polarization scrambler is important for keeping the transmission characteristic good.
However, to perform the phase adjustment of the high-speed electro-optic polarization scrambler, it was conventionally inevitable to send out an optical signal from an optical transmitter having the high-speed electro-optic polarization scrambler, perform a transmission characteristic measurement by the optical signal after transmitted over a long distance, for instance about 10000 km, and evaluate the transmission characteristic. Thus, there was a problem that the phase adjustment of the high-speed electro-optic polarization scrambler was difficult.